The charge was a major blow to TCF's quarterly earnings, which totaled $9.3 million, or 6 cents per diluted common share. That was down from $32.2 million in the same period a year earlier.

"While the third-quarter results were not what we anticipated due to the impact of the adoption of the clarifying regulatory guidance, we believe we are on the right track as we work through the remainder of 2012 and move forward with our strategy into 2013," TCF Chief Executive Officer William Cooper said in a news release.

But affected banks have countered by arguing that many of the borrowers in question have continued to pay and will continue to do so. For example, some people file for bankruptcy in order to discharge medical bills, but they keep paying their mortgages.

In addition to taking a charge, TCF moved $103.2 million of mortgage loans to nonaccrual status in order to comply with the guidance. More than 80% of those loans are first mortgages, and more than 90% of them were less than 60 days past due as of Sept. 30, according to the company.

During a conference call with analysts, Cooper, TCF's often-outspoken CEO, was restrained in his comments about the regulatory guidance. But he said that the directive will alter how banks deal with borrowers who have been through bankruptcy, making banks more likely to pursue foreclosures.

"There's certainly less incentive to work with a customer in terms of loan concessions and so forth, given that the loan has already been written down," Cooper said.

Chief Financial Officer Mike Jones said that in future quarters TCF expects to recover the charges it took. But in response to an analyst's question, he declined to offer a specific timetable. "The vast majority of these loans were current and have been current for a long time," Jones said.

For the quarter, TCF, which has $17.5 billion of assets, reported $312 million in revenue. That was 6% higher than in the third quarter of 2011, on the strength of strong net interest income, which was up 14% from the year-earlier period.

The rising net interest income was driven by a net interest margin of 4.85% — a high margin compared with most other banks, but one that Cooper said he expects to decline somewhat in coming quarters.